From the time incandescent lamps were first invented by Thomas Edison, significant research has been conducted to develop a new light source. Currently, various kinds of lightings have been used for providing light on a dark area and illuminating an object. Such lightings are based on the generation of light by converting electrical energy into light energy and currently include, in general, incandescent lamps, mercury lamps, and fluorescent lamps. However, the above light sources have several disadvantages such as high power consumption and short durability requiring frequent replacement.
Further, as the significance of environmental issues is increasingly being recognized, incandescent lamps and mercury lamps may be classified as environment-threatening hazardous materials due to the use of mercury which is a human carcinogen. Those light sources are also problematic in that they require a large installation space, it is very complicated to install them, it is difficult to control their color, and their application in various fields is very limited because of their properties as point light sources.
Recently, various types of alternative light sources are being developed in order to overcome the above-mentioned problems. Representative examples include light sources using LED (light emitting diode) or OLED (organic light emitting diode). Since the above light sources utilize light having a longer wavelength than ultraviolet light, they are harmless to human beings, have a long life-span and are environmentally favorable. Further, they can be applied to various light sources including LCD backlights, room lamps, and the like.
However, because of the complicated production process and high production cost, white LEDs and white OLEDs have a fairly limited scope of application. In the case of white LED, while a great deal of research on developing photoexcitation light emitting sheets for converting a blue LED into a white light has been carried out, there have been problems with respect to its application to large area light sources due to its utilization of a point light source, as well as the generation of heat during operation. As for white OLED, it has received attention as a light source appropriate for human eyesight since it is a surface light source and has wide color coordinates, but still has problems in terms of development of raw materials and life-span.
In the fabrication of conventional photoexcitation light emitting sheets, several dispersion methods including ultrasonication dispersion, mechanical dispersion (stirrer, homogenizer, etc), electrostatic dispersion (dispersing agents, charge control agents, surfactants, etc) and the like have been used for uniformly dispersing and mixing light emitting materials in a solution, but homogeneous dispersion and mixing cannot be maintained by such methods. Further, even if homogeneous dispersion and mixing are achieved, it is impossible to form a uniform film when fabricating a light emitting sheet by conventional methods in the art including spin casting, screen printing, bar coating, doctor blade and the like, due to a difference in density and surface energy and local evaporation of a solvent. Thus fabricated light emitting sheet having a local difference in thickness exhibits non-uniform brightness and color coordinates, which is problematic for commercialization. Additionally, because the formation of such a non-uniform film causes a falling-off in color reproducibility of white and color light sources, it is not easy to regulate color coordination. Further, the fabricated light emitting sheets have low thermoresistance and are susceptible to morphological changes at high temperatures, resulting in limited applicability.